Power MOSFET devices or insulated gate bipolar transistor (IGBT) devices may be used as switching devices in electronic devices. Such switching devices require a low on-resistance in an operating state, a high breakdown voltage, and a high switching speed. Since the on-resistance and the breakdown voltage generally have a trade-off relationship, it is difficult to implement a switching device simultaneously satisfying a low on-resistance and a high breakdown voltage. Recently, it has been reported that, among power MOSFET devices, a MOSFET device having a superjunction structure including an alternating conductivity type drift layer can simultaneously realize a low on-resistance and a high breakdown voltage. However, as dopant concentrations of P-type and N-type regions in the alternating conductivity type drift layer are decreased (or as breakdown voltages are increased), a field oxide is damaged by a high voltage applied to a termination region surrounding the P-type and N-type regions, and therefore, the reliability of the MOSFET device may be degraded.